I'd like to patch some Windows PE file to show just simple MessageBox. Taking into account that the PE in question doesn't import the MessageBoxA function if I'm right that all i need to do is to add the user32.dll to the Imports Section with MessageBoxA function. And then just patch the code (using IDA or OllyDbg or sth. similar) adding a call instruction to the address of the newly added MessageBoxA function from Imports? If I'm right that after adding a new entry to the Imports the Windows Loader automatically map the MessageBoxA function physical address to the virtual PE address space making the PE possible to call the needed function?
callwon't help you. Depending on what the target is -- OllyDbg makes me assume x86 or x64, though -- you need to be aware of the calling convention(s) and pass the arguments in registers or on the stack accordingly. So it'd be somemovorpushbefore thecall, too.